Black Holes (Sec. 22.5, 6, 7, 8 in textbook)
22.5 Black Holes The mass of a neutron star cannot exceed about 3 solar masses. If a core remnant is more massive than that, nothing will stop its collapse, and it will become smaller and smaller and denser and denser. Eventually, the gravitational force is so intense that even light cannot escape. The remnant has become a black hole.
Black holes The radius at which the escape speed from the black hole equals the speed of light is called the Schwarzschild radius. The Earth’s Schwarzschild radius is about a centimeter; the Sun’s is about 3 km. Once the black hole has collapsed, the Schwarzschild radius takes on another meaning—it is the event horizon. Nothing within the event horizon can escape the black hole.
22.6 Einstein’s Theories of Relativity Matter tends to warp spacetime, and in doing so redefines straight lines (the path a light beam would take): A black hole occurs when the “indentation” caused by the mass of the hole becomes infinitely deep.
22.7 Space Travel Near Black Holes The gravitational effects of a black hole are unnoticeable outside of a few Schwarzschild radii—black holes do not “suck in” material any more than an extended mass would. Matter encountering a black hole will experience enormous tidal forces that will both heat it enough to radiate, and tear it apart: A probe nearing the event horizon of a black hole will be seen by observers as experiencing a dramatic redshift as it gets closer, so that time appears to be going more and more slowly as it approaches the event horizon. This is called a gravitational redshift—it is not due to motion, but to the large gravitational fields present. The probe, however, does not experience any such shifts; time would appear normal to anyone inside.
22.7 Space Travel Near Black Holes (cont.) Similarly, a photon escaping from the vicinity of a black hole will use up a lot of energy doing so; it cannot slow down, but its wavelength gets longer and longer
22.7 Space Travel Near Black Holes What’s inside a black hole? No one knows, of course; present theory predicts that the mass collapses until its radius is zero and its density is infinite, but it is unlikely that this actually happens. Our ignorance is due to the fact that the densities and temperatures in a black hole are so large that gravity and all the other forces should be unified as one--”quantum gravity.” Physics has no theory for such a state. Until we learn more about what happens in such extreme conditions, the interiors of black holes will remain a mystery.
22.8 Observational Evidence for Black Holes Black holes cannot be observed directly, as their gravitational fields will cause light to bend around them. Otherwise you could possibly see the black hole as a dot against the bright background star.
22.8 Observational Evidence for Black Holes The existence of black-hole binary partners for ordinary stars can be inferred by the effect the holes have on the star’s orbit, or by radiation from infalling matter. In the case shown, a giant B star is observed, using the Doppler effect, to be orbiting something that is too faint to be seen . X-ray emission is also observed from the system, presumably an accretion disk around some kind of compact remnant. White dwarf? Neutron star? Black hole? Theoretically, the distinction should be the mass of the object. If larger than about 10 solar masses, the object should have too much gravity to become a white dwarf or a neutron star. Unfortunately it is difficult to obtain masses even for “normal” stars.
22.8 Observational Evidence for Black Holes This bright star has an unseen companion that is a strong X-ray emitter called Cygnus X-1, which is thought to be a black hole. Cygnus X-1 is a very strong black-hole candidate: • Its visible partner is about 25 solar masses. • The system’s total mass is about 35 solar masses, so the X-ray source must be about 10 solar masses. • Hot gas appears to be flowing from the visible star to an unseen companion. • Short time-scale variations indicate that the source must be very small.
22.8 Observational Evidence for Black Holes There are several other black-hole candidates as well, with characteristics similar to those of Cygnus X-1. The centers of many galaxies contain supermassive black hole—about 1 million solar masses. Recently, evidence for intermediate-mass black holes has been found; these are about 100 to 1000 solar masses. Their origin is not well understood.
Discovery 22-1: Gravity Waves: A New Window on the Universe General relativity predicts that orbiting objects should lose energy by emitting gravitational radiation. The amount of energy is tiny, and these waves have not yet been observed directly. However, a neutron-star binary system has been observed (two neutron stars); the orbits of the stars are slowing at just the rate predicted if gravity waves are carrying off the lost energy. However this is not a direct detection of gravity waves, which are the “holy grail” of relativity. This figure shows LIGO, the Laser Interferometric Gravity-wave Observatory, designed to detect gravitational waves. It has been operating since 2003, but no waves have been detected yet. That was two years ago… What is the current status? If there had been a detection, we would know about it!
Summary of Chapter 22 • Supernova may leave behind a neutron star. • Neutron stars are very dense, spin rapidly, and have intense magnetic fields. • Neutron stars may appear as pulsars due to the lighthouse effect. • A neutron star in a close binary may become an X-ray burster or a millisecond pulsar. • Gamma-ray bursts are probably due to two neutron stars colliding or hypernova.
Summary of Chapter 22 (cont.) • If core remnant is more than about 3 solar masses, it collapses into black hole. • We need general relativity to describe black holes; it describes gravity as the warping of spacetime. • Anything entering within the event horizon of a black hole cannot escape. • The distance from the event horizon to the singularity is called the Schwarzschild radius. • A distant observer would see an object entering black hole subject to extreme gravitational redshift and time dilation. • Material approaching a black hole will emit strong X-rays. • A few such X-ray sources have been found and are black-hole candidates.
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